Implants with threads over a substantial portion of their external surface, whether for self-tapping insertion or otherwise, implants that are externally unthreaded and implants that are both externally ribbed and externally threaded, are generally known and commercially available. Externally-threaded implants usually have an unthreaded portion at the proximal end of the implant that is commonly referred to as the neck portion, with the remainder of the external surface substantially threaded to or near to the distal end of the implant. Where present, self-tapping features serve the function of cutting threads in a cylindrical passage formed in the jawbone tissue of a person to receive the implant as the externally-threaded implant is rotated to a depth that places the neck of the implant above, at or just below the crest of the jawbone.
Self-tapping insertion of an externally-threaded implant is accomplished by forming, as by machining, one or more grooves on the sidewall extending upwardly from the distal end parallel to the longitudinal axis of the dental implant and through at least one full diameter external thread. These grooves create cutting edges that function to scrape off bone chips during threading of the implant into the cylindrical hole prepared in the bone tissue. The grooves also provide cavities with adequate volume to contain the bone tissue material to allow full seating of the implant.
Some self-tapping implants also provide a through-hole connecting two channels on opposite sides of the implant to provide additional cavity space to harbor bone chips and to further stabilize the implant once bone regeneration has occurred.
Self-tapping insertion of the implant has proven advantageous from a time-saving standpoint (Fribert B. et al.; JOMI 1992; 1:80-84) by reducing surgical time by 3 minutes or more per implant. Self-tapping insertion of the externally-threaded implants also improves the initial stability needed for direct bone attachment following a healing period, referred to as osseointegration, by creating a more intimate contact with the bone than placement following use of a bone tap surgical instrument. This more intimate initial fit has also been demonstrated to result in an increased percentage of bone attachment to the implant surface after healing (Cook S. et al. J Oral Implant 1993: 4:288-294). For self-tapping insertion to be effective in dense bone, the cutting edges created by the grooves through the distal threads must be sharp enough to shave bone chips. Roughening the implant surface by grit-blasting, or by grit-blasting followed by acid etching, or by grit blasting followed by coating the surface of the implant with Titanium Plasma Spray (TPS) or with a bio-reactive material such as Hydroxylapatite (HA), rounds these cutting edges, decreasing the cutting efficiency of the self-tapping features. This can necessitate increasing the torque forces needed to insert the self-tapping implant in dense bone to the point that damage may occur to the wrench-engaging feature in the proximal portion of the implant, resulting in failure to seat the implant fully in the bone chamber.
Self-tapping screw implants are usually machined from a biocompatible metal of suitable strength such as commercially-pure titanium or from medical grade titanium alloy. The selection of Grade 1 or 2 commercially-pure (CP) titanium, with tensile strengths lower than Grade 3 or 4 CP titanium or titanium alloy (6AI/4V), may preclude the incorporation of through-holes because of the lower tensile strength. Such lower tensile strength may also limit the density of bone that the implant can self-tap into because of the lower resistance to distortion of the wrench-engaging surfaces at or near the proximal end of the implant as higher torque forces are required to cut through dense bone.
Some self-tapping screw implants are sold with a machined surface (Nobelpharma, Inc. implants and others (Core-Vent Corporation's SCREW-VENT implant) are further treated after machining by washing in dilute HF acid to remove loose titanium particles and other contaminants. Acid etching creates pits on the surface of the implant, similar in surface roughness to the untreated machined surface. Machined and etched surfaces are relatively smooth compared to grit blasted, TPS-coated or HA coated surfaces.
Some commercially available self-tapping screw implants have their threaded external surfaces treated to increase surface roughness while maintaining the neck portion relatively smooth by leaving it with a machined or etched surface or by mechanically polishing the surface. A smooth neck portion promotes mucosal tissue health if it becomes exposed to the mucosa. The texture of the implant's external surface is increased in roughness by grit-blasting with a variety of bio-compatible particles such as titanium oxide (Astra implants), aluminum oxide (CORE-VENT implant, pre-1986) or tri-calcium phosphate (Screw-Vent, post 1997). The degree of roughness can be varied by varying the size and hardness of the abrasive particles, and by varying the force and duration of the blasting procedure. Some screw implants, after machining, are grit-blasted to roughen the surface preparatory to applying a coating of either Titanium Plasma Spray, (as in the Bio-Vent, Friatec, Straumann ITI implants), which provides both a rough and porous surface, or a coating of a bio-active material such as Hydroxylapatite (HA: STERI-OSS, SCREW-VENT and MICRO-VENT implants). HA may be densely applied and of high crystallinity, which produces a surface roughness approximating that of a small-particle grit blasted surface, or may be less dense and/or less crystalline, which produces a surface roughness that could match or exceed that of TPS coating or large particle grit-blasting.
Surface Texture and Material Effect Bone Attachment
Studies have documented increased removal torques with implants having increased surface roughness (Carlisson, Albrektsson et al., JOMI 1988: Vol. 3), and other studies have shown increased bone attachment to rougher surfaces (Buser: J Biomet Mater Re 1991: Vol. 25). A study comparing bone attachment to HA coated and machined surfaces demonstrated a faster, more complete attachment to the HA surface in the critical, early healing period (Gottlander M., Albrektsson T.: JOMI 1991: Vol. 4).
At the cellular level, one study (Bowers, K. T. et al.: JOMI 1992:7(3) p. 301-310) found higher levels of attachment of osteoblast-like cells to surfaces with random roughness created by grit-blasting and acid etching compared to parallel grooved surfaces similar in appearance to a machined surface, created in this study by grinding the surface with 120 and 60 gauge grit. This was true despite the fact that the surface roughness of the grooved surface, created by grit polishing, was rougher than that produced by the acid etch procedure, indicating that random roughness promotes bone attachment better than parallel or concentric grooves.
Another study comparing bone attachment strength to HA and to a rougher grit-blasted surface documented 77% increase in torsional strength for the HA coated surface, indicating that HA is bio-active and created a chemical as well as mechanical bond with the bone.
Studies have measured the differences in surface roughness of commercially-available implants using scanning microscope profilometry Wennerberg, Ann, et al.,Design and Surface Characteristics of 13 Commercially Available Oral Implant Systems, JOMI, Vol. 8, No. 6, pages 622-633 (1993)!, and determined that the machined Branemark surface was the smoothest with an average difference, called R.sub.t, of about 10 microns between the peaks and valleys of the surface texture. The Wennerberg article defines the term R.sub.t, at page 623, to mean the maximum peak-to-valley height of the profile of the implant surface and the assessment length, measured in micrometers. The assessment length is the length of the implant subjected to analysis.
Using the R.sub.t measurement standard as set forth in the Wennerberg article, the acid etched surface of a SCREW-VENT implant, made from commercially-pure titanium, measured an average of about 10 microns. The grit-blasted and acid-etched surface of the titanium alloy Core-Vent has an average R.sub.t value of about 18 microns. The TPS coated surface of the IMZ implant had an R.sub.t value of about 25 microns. HA coated surfaces of four implants were measured in the Wennerberg study, which reported an R.sub.t value of about 18 microns for Calcitek's highly dense, crystalline HA surface. Several other HA coatings, which are more porous and less crystalline, had R.sub.t values up to about 40 microns.
Regardless of the smoothness of the HA surface, it is unsuitable material for coating the neck portion of an implant that may become exposed to oral mucosa when crestal bone recession occurs around the top of the implant. Coated surfaces so exposed to the oral environment either increase the attachment of dental plaque, or dissolve, exposing the rough, grit-blasted undersurface, which also increases the attachment of dental plaque. Plaque around the exposed neck of an implant causes adverse mucosal tissue reactions and ultimately increased bone loss, just as with natural teeth. Crestal bone cratering and associated soft tissue complications have been reported with Calcitek's non-threaded cylinder implants that have a dense, relatively smooth, HA coating all the way to the top of the implant (Johnson; Calif. Dental Journal, JOMI 1994, Special Supplement).
Exposure of the machined surface of the neck of the implant above the crest of the bone routinely occurs with the Branemark machined implants, but long-term studies do not indicate that such exposure of the machined surface to mucosal tissue attracts dental plaque any more than with natural teeth. Oral hygiene can be maintained on this relatively smooth surface, minimizing soft tissue complications. A VA multicenter study reported an average of 0.75 to 1.5 mm of bone loss 6 months after exposure for both acid etched (smooth) and for HA-coated implants with a 0.5 mm acid-etched neck, exposing HA coating to mucosal tissue.
The clinical complications of exposure of the rough TPS coating into the gingival area have been documented in a clinical study of 54 ITI implants where all the implants osseointegrated. However, within 3 years, 3 implants exhibited recurrent per-implant infections and were classified as late failures (Buser JOMI 1991, Vol. 4).
Implant manufacturers, recognizing the potential benefits of the bio-active HA coatings and the rougher surfaces of the TPS coatings or large particle grit blasting with or without subsequent acid etching, have attempted to limit the complications associated with exposure to these rough or bio-reactive surfaces to the oral cavity by maintaining an uncoated machined (3i), or acid etched (Core-Vent) metal portion extending down from the top of the TPS or HA coated implant a distance ranging from 0.5 mm to over 3 mm in length. The longer the smooth neck, the more extensive the bone loss, but the shorter the smooth neck, the more likely the exposure of mucosal tissue to the roughened or coated external implant surface.